<!DOCTYPE html>
<html lang="en" xmlns:v-on="http://www.w3.org/1999/xhtml">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
<div id="app">
  {{counter}}<br>
  <button v-on:click="add">+</button>
  <button @click="sub()">-</button>
</div>
<script src="../js/vue.js"></script>
<script>
  const vm=new Vue({
    el:'#app',
    data:{
      counter:0
    },
    methods:{
      add(){
        this.counter++
      },
      sub(){
        this.counter--
      }
    }
  })
</script>

</body>
</html>